Electron multiplier for ultra high frequencies



June 14, 1949. c. c.

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IN ELECTRON-VOL ATTORNEY Patented June 14, 1949 ELECTRON MULTIPLIER FORULTRA HIGH FREQUENCIES Christian C. Larson, Fort Wayne, Ind., assignor,by mesne assignments, to Farnsworth Research Corporation, a corporationof Indiana Application April 14, 1945, Serial No. 588,315

6 Claims.

This invention relates to electron multipliers and particularly toelectron multipliers arranged for amplifying ultra-high frequencies.

It is well known that electron multipliers, wherein a primary electroncurrent is multiplied by secondary electron emission, have a definiteupper frequency limit. This occurs at ultra-high frequencies when thereciprocal of the frequency of the input signal is of the order of theelectron transit time between two successive stages. One explanation forthe cut-off frequency of electron multipliers is the so-called electrontransit time spread. The primary, as well as the secondary, electronspassing between the secondary electron emissive stages of an electronmultiplier have a certain initial velocity range or spread which is ofthe order of a few electron-volts. At very high, frequencies this spreadof the electron velocities masks the signal to be amplified because theoutput has a direct current component, as will be more fully explainedhereinafter. Furthermore, the difierences in the paths of differentelectrons traveling between two successive secondary electron emissivestages also causes a transit time spread. This eifect, however, can beminimized ducing the distance between two successive mul- I tiplyingstages. However, there are obvious limits to these expedients. In viewof constructional difliculty the distance between two successive stagescan not be made too short. Furthermore, cold emission from closelyspaced edges of the plates of the multiplier makes it impossible toincrease the accelerating potential beyond a certain limit. Therefore,it would be Very desirable to decrease the electron transit time spread,thereby to raise the upper frequency limit of the electron multiplier.

It is an object of the present invention, therefore, to provide anelectron multiplier having a higher cut-off frequency than priorelectron multipliers.

Another object of the invention is to provide an electron multiplierwhich has a reduced electron transit time spread thereby to raise theupper frequency limit thereof.

In accordance with the present invention, there is provided an electronmultiplier comprising a plurality of secondary electron emissiveelectrodes and a source of primary electrons. Means are provided fordirecting primary electrons from the source towards the first one of thesecondary electron emissive electrodes, as well as means for directingsecondary electrons liberated from each of the electrodes towards thesucceeding electrode. Means are also provided for controlling the numberof electrons passing between the electrodes in accordance with an inputsignal. Further means are provided for passing only electrons within apredetermined velocity range between successive electrodes and means forcollecting the electrons from the last secondary electron emissiveelectrode to derive an amplified output signal. Thus, the electrontransit time spread is reduced and the frequency cut-off of themultiplier is raised.

For a better understanding of the invention, together with other andfurther objects thereof, reference is made to the following description,taken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is a schematic representation of two secondary electron emissivestages and electron clouds therebetween representative of a signal to beamplified.

Fig. 2 is a curve illustrating the velocity distribution of secondaryelectrons; and

Fig. 3 is a sectional view of an electron multiplier and an associatedelectric circuit embodying the present invention.

Referring now more particularly to Fig. l of the drawing, there areshown two secondary electron emissive stages I and 2 of an electronmultiplier having electron clouds schematically indicated at 3. Electronclouds 3 may represent a signal to be amplified. We may assume that theelectrons of which electron clouds 3 are composed have been liberated,for example, by the impact of primary electrons, not shown, from stage Iand are accelerated towards stage 2. When the electrons of clouds 3impact stage 2, they will liberate further secondary electrons. Theelectrons of each electron cloud 3 have a velocity distribution of thetype illustrated in Fig. 2. In Fig. 2 curve 4 shows the number ofelectrons plotted against their initial velocity expressed inelectron-volts. When the frequency of the input signal is increased,electron clouds 3 will be more closely spaced together. Now, the fasterelectrons of any electron cloud, such as 5, will overtake the slowerelectrons of the preceding electron cloud, such as 6. Accordingly,electron clouds 3 are no longer as sharply separated as illustrated inFig. 1 and, hence, the alternating current component of. the signaloutput will be-reduced so that at least part of the input signal islost.

This eflfect is known as the electron transit time.

spread and results in a definite upper frequency limit beyond which theelectron multiplier. can not be operated. Experiments have shown that atfrequencies of the order of 50 megacycles the frequency response of anelectron multiplier shows an appreciable deviation from its staticresponse, that is, the response at low frequencies. This value, however,depends upon the design of the multiplier and a number of other factorssuch as the transit time between individual multiplier stages and thelike; The theoretical cut-off frequency ismuch higher, being of theorder of 1000 megacycles.

In accordance with the present invention the electron transit timespread is reduced by passing only electrons between the multiplyingstages of an electron multiplier which have a predetermined velocityrange. An electron multiplier embodying the' present invention isillustrated in Fig. 3. Electron multiplier 10 comprises evacuatedenvelope H and an electron source or primary emitter, such as indirectlyheated cathode 1-2. It is to be understood that, for example, aphotocathode may also be used as a source of primary electrons. Electronmultiplier 10 further comprises control grid I3 and a plurality ofsecondary electron emissive stages or electrodes 14, l; l6, l andelectron collector '20. The control grid l'3- is the means forcontrolling the number of primary electrons from the primary emitter tothe first electrode l4.

Electron multiplier l0 preferably is of the magnetic type and providedwith coil 2| supplied with direct current from a suitable source suchas, for example, battery 22. Coil 2! is arranged to create amagneticfield, the lines of force of which pass perpendicularly through the:plane of the drawing.

For the purpose of supplying operating potentials tothe electrodes ofmultiplier I0, there is provided a voltage source such as, for example,battery 23 connected acrosspotentiometer 24 and having its positiveterminal grounded". By means of lead25, cathode I2 is maintained at apotential that is negative against ground. Lead 26 connected throughresistor 21 to a suitable tap of potentiometer 24 normally keeps controlgrid [3 at a potent al that is a few volts negative against that ofcathode l2. Electron multiplying stages [4' to I! are kept atincreasingly positive potentials with respect to cathode l2. Electroncollector 20' is connected by lead '28 through load resistor 30 toground and, hence, has a more positive-potential than multiplyingstage". The input signal connected to terminals 3| may be impressedbetween leads 25 and 26 through condensers 32. Hence, the input signalis impressed between control grid l3 and cathode l2. However; it is tobe understood that the input signal may also be impressed, for instance,between. any two electron multiplyin stages I4, I5, 16 or H.

In accordance with the present invention a velocity filter is providedbetween electron source [=2 and first electronmultiplying stage l4, aswell as between every two succeeding multiplying stages. Preferably, thevelocity filter is of the magnetic type and includes coil 2| forgenerating a constant magnetic electron deflecting field. The magneticvelocity filter further includes shields 33, 34, 35, 36 and 31 arrangedbetween cathode l2 and first multiplying stage 14, as well as betweenevery two succeeding multiplying stages. Shields 33 to 3 1' are eachprovided with a suitable slot or aperture 38. Shield 33, for instance,is maintained by means of a suitable tap of potentiometer 2.4 at apotential intermediate between that of cathode l2 and of multiplyingstage Id. Shield 36 in turn is kept at a potential between'that of.multiplying stages I l and i5, and similarly shields 35, 36 and 31 aremaintained at a potential intermediate between that of their associated.multiplying stages.

By virtue of the magnetic field created by coil 21* the primaryelectrons developed by cathode l2, as well as the secondary electronsliberated from each multiplying stage 14 to H, are deflect'edinaccordance with their velocities. Thus, the radius of curvature of theelectron paths is directlyproportional to the velocity of the electrons.Slots 38 in shields 33 to 3T are arranged in such a manner that onlyelectrons within a predetermined velocity range are able to pass from.one multiplying stage to the succeeding stage.

Referring again to Fig. 2, it is, for instance, feasible to select thoseelectrons which have a velocity corresponding to between two and threeel'ectron-voltsfor passing them between two succeeding' multiplyingstages. All other electrons, that is, those-which have a velocity oflessthan two electron-volts or more than three electron volts are rejected,that is, they are collected by one of the shields 33 17031. It is ofcourse to be understood that any other velocity range may be selected;The velocity of secondary electrons depends upon many factors, such asthe energy of the primary electrons and the field gradient in theneighborhood of the secondary electron emissive surface. Hence, theshape of curve 4 as well as the values given on the abscissa arerepresentative only of certain conditions. It will be obvious that thenumber of secondary electrons selected for passage between successivemultiplying stages should be larger than the number of primary electronswhich have liberated the secondary electrons, because otherwise thedevi'cewill no longer function as a multiplier of electrons. However,inspection of Fig. '2 will show'that the-velocity spread of theelectrons can be. considerably'reduced without unduly reducing thenumber of electrons allowed to pass between the multiplying stages.

The operation of. electron multiplier I0 is conventional' and,therefore, a short explanation thereof will'besufiicient. The primaryelectrons developed by cathode [2 are attracted by secondary electron,emissive stage I'd under the combinedinfiuence of. the magnetic fielddevelopedby coil 2| and the electric potential between cathode l2 and.multiplying stage 14. Only electrons within a predetermined velocityrange are able to. reach multiplying stage 14. In a similar manner,secondary electrons are liberated from each of the succeedingmultiplying. stages l5, l6 and H. Screens 34., 35, 36. and. 31. preventelectrons outside. the predetermined velocity range. from reaching. thesucceeding stage. The multiplied electron current is collected byelectron collector 20. The. output signal. is, developed across loadresistor 30 and may be obtained from output lead 40.

Although a magnetic velocity filter as described hereinabove ispreferred, it is also feasible to utihas a velocity filter of theelectric type. Such an electric velocity filter has been described, forexample, in United States Patent 2,271,985 to Morton. In this case theelectrons are deflected in accordance with their velocities by means ofan electric field. However, the advantage of a magnetic velocity filteris that a uniform magnetic field created by one coil, such as coil 2!,is sufficient to deflect the electrons from all multiplying stages ofthe multiplier.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron multiplier system comprising a plurality of secondaryelectron emissive elec trodes, a primary emitter of primary electronsmounted adjacent said electrodes, voltage means coupling said primaryelectron emitter and the first one of said electrodes for directingprimary electrons from said primary emitter towards the first one ofsaid electrodes, means magnetically coupling said electrodes fordirecting secondary electrons liberated from each of said electrodestowards the succeeding electrode, means coupled to one of said electronemitters for controlling the number of electrons passing between saidelectrodes in accordance with an input signal, means including membersmounted between said electrodes for passing only electrons within apredetermined velocity range between successive electrodes, and meansmounted adjacent the last one of said electrodes for collecting theelectrons from the last one of said electrodes to derive an amplifiedoutput signal, thereby to reduce the electron transit time spread and toraise the frequency cut-ofi of said multiplier.

2. An electron multiplier system comprising a plurality of secondaryelectron emissive electrodes, a primary emitter of primary electronsmounted adjacent said electrodes, voltage means coupling said primaryelectron emitter and the first one of said electrodes for directingprimary electrons from said source towards the first one of saidelectrodes, means magnetically coupling said electrodes for directingsecondary electrons liberated from each of said electrodes towards thesucceedin electrode, means coupled to one of said electron emitters forcontrolling the number of electrons passing between said electrodes inaccordance with an input signal, a velocity filter device includingmembers mounted between said electrodes for passing only electronswithin a predetermined velocity range between successive electrodes, andmeans mounted adjacent the last one of said electrodes for collectingthe electrons from the last one of said electrodes to derive anamplified output signal, thereby to reduce the electron transit timespread and to raise the frequency cut-oif of said multiplier.

3. An electron multiplier system comprising a plurality of secondaryelectron emissive electrodes, a source of primary electrons mountedadjacent said electrodes, voltage means coupling said source and thefirst one of said electrodes for directing primary electrons from saidsource towards the first one of said electrodes, means coupled to saidsource for controlling the number of electrons passing between saidelectrodes in accordance with an input signal, a magnetic velocityfilter device including shields mounted between said electrodes forpassing only electrons within a predetermined velocity range betweensuccessive electrodes, and means mounted adjacent the last one of saidelectrodes for collecting the electrons from the last one of saidelectrodes to derive an amplified output signal, thereby to reduce theelectron transit time spread and to raise the frequency cut-off of saidmultiplier.

4. An electron multiplier system comprising a plurality of secondaryelectron emissive electrodes, a source of primary electrons mountedadjacent one of said electrodes, voltage means coupling said source andthe first one of said electrodes for directing primary electrons fromsaid source towards the first one of said electrodes, means mountedadjacent said source for controlling the number of electrons passingbetween said electrodes in accordance with an input signal, a magneticvelocity filter device including a constant magnetic field producingmeans mounted adjacent said electrodes and a plurality of aperturedshields mounted between said electrodes for passing only electronswithin a predetermined velocity range between successive electrodes, andmeans mounted adjacent the last one of said electrodes for collectingthe electrons from the last one of said electrodes to derive anamplified output signal, thereby to reduce the electron transit timespread and to raise the frequency cut-01f of said multiplier,

5. An electron multiplier system comprising a plurality of secondaryelectron emissive electrodes, a source of primary electrons mountedadjacent an end one of said electrodes, a unidirectional power supply,means including said power supply coupling said source and the first oneof said electrodes for directing primary electrons from said sourcetowards the first one of said electrodes, means mounted in the path ofsaid primary electrons for controlling the number of electrons passingbetween said electrodes in accordance with an input signal, a magneticvelocity filter device including means surrounding said electrodes forcreatin a constant magnetic field extending between said source and saidelectrodes and a plurality of shields each being provided with a slot,one of said shields being mounted between said source and said firstelectrode and the remaining shields being mounted between succeedingelectrodes in such a manner that only electrons within a predeterminedvelocity range can prass through the slot of each of said shields, andmeans mounted adjacent the last one of said shields for collecting theelectrons from the last one of said electrodes to de- "ive an amplifiedoutput signal, thereby to reduce the electron transit time spread and toraise the frequency cut-off of said multiplier.

6. An electron multiplier system comprising a plurality of secondaryelectron emissive electrodes, a source of primary electrons mountedadjacent the first one of said electrodes, voltage means coupling saidsource and the first one of said electrodes for directin primaryelectrons from said source towards the first one of saidelectrodes,-means including a coil surrounding said electrodes toproduce a constant magnetic field for directing secondary electronsliberated from "7 each of said electrodes towards the succeedingelectrode, means mounted between said source of primary electrons andthe first one of said electrodes for controlling the number of electronspassing between said electrodes in accordance with an input signal, amagnetic velocity filter device including a plurality of aperturedshields mounted between said source and said first electrode and betweensucceeding electrodes for passing only electrons within a predeterminedvelocity range between successive electrodes, and means mounted beyondthe last one of said shields for collecting the electrons from the lastone of said electrodes to derive an amplified output signal, thereby toreduce the electron transit time 5' 2,264,269

spread and to raise the frequency cut-off of said multiplier.

CHRISTIAN C. LARSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS 10 Number Name Date 1,903,569 Jarvis Apr. 11, 19332,138,928 Klemperer Dec. 6, 1938 2,147,756 Ruska Feb. 21, 1939 2,227,062Brett Dec. 31, 1940 Banks Dec. 2, 1941

